The present invention relates generally to systems employing time division multiplexing for gathering data from two or more sensors and to sensors and modulators used in such systems. Throughout this specification, the term "sensor" will be used to denote a device for detecting a physical phenomenon under test and directly converting the detected signal to a modulated sensor output signal. Throughout this specification, the term "modulator" will be used to denote the broad class of devices which includes both "sensors" and devices (used in conjunction with "sensors") that do not directly detect a physical phenomenon under test, but instead receive the output signal of a sensor and convert such sensor output signal into another type of modulated signal suitable for transmission. More particularly, the present invention relates to modulators (and sensors) capable of modulating the intensity of an interrogating light signal in response to variations in an incident signal, and to time division multiplexing data gathering systems comprising one or more arrays of such modulators (or sensors).
The invention utilizes the effect known as "evanescent field coupling," whereby a portion of the electromagnetic energy injected into an optical fiber segment is coupled to an adjacent optical fiber segment, with the intensity of the coupled portion depending on the separation between the two fiber segments. Signals incident on the modulator (and sensor) disclosed herein cause displacement of a fiber segment through which an interrogating light pulse propagates relative to another fiber segment to produce a coupled return signal in the latter segment whose intensity depends on the separation between the two segments at the instant the interrogating pulse passes through the former segment.
In gathering data from a large number of sensors, two general types of methods have been used. In the first, a wire pair is run from each sensor to a data recording unit. In the second, some form of multiplexing is used so that data from many sensors is impressed on a data bus consisting of a single wire pair, coaxial cable, or optical cable. In practicing the second type of method, a saving in wire (or other data transmission material) and space for cable runs is realized. However, in practicing conventional embodiments of such type of method, a significant amount of electronic equipment has generally been required to digitize and encode information from each sensor input location. In practicing the method of the present invention, the advantages of multiplexing are obtained, and the amount of electronic equipment required at each sensor-data bus interface is reduced.
One important application for the present invention is in the field of marine seismology. In marine seismology the most commonly employed technique for obtaining geophysical data is the reflection seismograph technique which typically requires the use of a large number of hydrophone arrays connected to form what is known as a "marine streamer." The marine streamer is towed behind a seismic vessel. The individual hydrophones may be made up of a piezoelectric element which converts acoustic signals to electrical signals. Marine streamers typically use electrical cables to transmit such electrical signals from the submerged hydrophones to instruments which display or record these signals on board the seismic vessel.
A typical marine streamer may have 200 hydrophone arrays. Each array may be 15 meters long and may be made up of 17 hydrophones in parallel. Such a marine streamer would be three kilometers long, would have 3400 hydrophones, and would require at least 400 wires running the length of the electrical cable to connect each array with the vessel. In addition, other wires would be needed for depth measurement, control, and other purposes. The cable diameter necessary for accommodating such a large number of wires would be about 3 inches.
Longer marine streamers are desirable, but extension of the apparatus commonly used in the art would be awkward because of the need for increased cable diameter to accommodate such increased length. Another approach that has been taken utilizes a digital streamer. In this type of system, the data from each array is digitized, multiplexed, and then transmitted down a data bus to instruments on board the seismic vessel. This digital streamer approach, although allowing smaller diameter streamers, results in a more expensive system in the water, and usually requires relatively large diameter electronics packages positioned at various locations along the streamer which act as noise sources as the streamer is dragged through the water.
Systems have been proposed which employ optical transducers for converting acoustic vibrations incident on a device such as a hydrophone or geophone into optical signals, and then into electrical signals. Such systems would replace the conventional piezoelectric transducers with generally more complex fiber optic transducers. The problem of transmitting many such signals down the streamer remains the same.
One method of alleviating the problem of increased cable diameter is through the use of optical fibers in place of the electrical wiring. Fiber optic systems have been proposed which convert incident acoustic vibrations into optical signals and maintain such optical signals in optical form for transmission. Such previously proposed systems employ couplers and lossy sensors which severely limit the number of signals which practically can be handled.
U.S. Pat. No. 4,071,753, issued Jan. 31, 1978 to Fulenwider et al. discloses several embodiments of an optical transducers which comprises a source of optical power connected to one end of an input optical fiber, means for varying the portion of optical power coupled between the other end of the input optical fiber and one end of an output optical fiber in response to oscillatory mechanical motion indicative of incident acoustic vibrations. One embodiment of the Fulenwider et al. transducer, discussed at column 6, lines 28 through 58, utilizes the effect known in the art as "microbending" by applying a varying degree of bending to an optical fiber to cause light propagated through the fiber to radiate away from the fiber in the vicinity of the bend, thus decreasing the amount of optical power transmitted through the bend as a function of its radius of curvature. Fulenwider et al., however, neither discusses the effect of evanescent field coupling between cores of adjacent optical fibers nor discloses any optical transducer utilizing such effect.
Another type of fiber optic transducer mechanism relies on phase modulation in a single mode fiber immersed in a fluid. The phase modulation in such a system is due to changes in the optical length of the fiber induced by sound waves propagating in the fluid. See, for example, J. A. Bucaro, H. D. Dardy, and E. F. Carone, "Fiber-optic hydrophone", Journal Acoustic Society of America, Vol. 62, No. 5, pp. 1302-1304, 1977.
A related optical transducer system is disclosed in U.S. Pat. No. 4,313,185 issued Jan. 26, 1982 to Chovan. Chovan discloses a hydrophone system comprising a first and a second single mode optical fiber and means for coupling light from the first fiber to the second fiber and from the second fiber to the first fiber. The optical length of the optical coupling path between the two fibers is modulated in response to acoustic vibrations incident on the fibers. The phase and frequency of light traversing the optical coupling path will vary with the optical length of the path and the rate of change thereof, respectively. Chovan neither discusses the effect of evanescent field coupling between cores of adjacent optical fibers nor discloses any optical transducer utilizing such effect.
U.S. Pat. No. 4,295,738, issued Oct. 20, 1981 to Meltz et al., discloses a fiber optic strain sensor comprising a single mode optical fiber having two or more cores positioned in a common cladding. At one end of the fiber, one of the cores is illuminated, and as the light propagates down the fiber, some light is coupled to adjacent cores due to crosstalk. Detector means are provided at the other end of the fiber for measuring the intensity of light emerging from each core. A pressure change or strain acting on the fiber causes a change in the indices of refraction of the cores and cladding and in the dimensions of the fiber. This results in a change in the crosstalk between the cores and thus in a change in the intensity of light emerging from the cores.
The Meltz et al. apparatus has limited sensitivity due to the placement of several cores within the relatively rigid structure of a single fiber. This structure de-emphasizes the effect of possible changes in core separation which may result from the application of strain or pressure to the fiber. Also the Meltz et al. apparatus is limited in that it requires a single mode optical fiber, and could not be used with a multi-mode optical fiber.
A different type of optical transducer system, which may be suitable in a hydrophone for some applications, is disclosed in U.S. Pat. No. 4,268,116, issued May 19, 1981 to Schmadel et al. The Schmadel et al. method and apparatus produces a modulated light signal in a single mode clad optical fiber by varying the frequency and/or phase of a narrow band of light reflected back to its source by an optical grating, by sliding the optical grating relative to the fiber near its core. The Schmadel et al. apparatus depends on the phenomenon of Bragg reflection by the optical grating. The present invention, however, requires no such optical grating and does not utilize the Bragg reflection phenomenon.
The effect of "evanescent field coupling," whereby a portion of the electromagnetic energy in an optical fiber is coupled to an adjacent optical fiber, is well understood. The coupling effect occurs between multi-mode fibers as well as between single-mode fibers. It has been recognized that the magnitude of power so coupled between two fibers depends on the separation between them. It also has been recognized that the effect could, in principle, be utilized in a transducer to produce an intensity-modulated signal in response to a variation in the separation between two optical fibers. See, for example, S. K. Sheem and J. H. Cole, "Acoustic Sensitivity of Single-Mode Optical Power Dividers", Optics Letters, Vol. 4, No. 10, p. 322 (1979). The apparatus of the present invention, however, utilizes the evanescent field coupling effect in a manner not previously suggested in the art.